The present invention relates to a novel crystal modification (xe2x80x9cxcex2 modificationxe2x80x9d) resistant to dyeing of the dye of the formula I 
which in the X-ray diffraction pattern (Cu Kxcex1 radiation) exhibits lines at the following diffraction angles 2xcex8 (xc2x0):
high-intensity lines: 7.15, 10.25, 25.9,
medium-intensity lines: 16.9, 19.5, 20.1, 21.85, 22.65, 23.4, 25.2, 28.5, 32.45.
The X-ray diffraction pattern of the xcex2 modification resistant to dyeing, which was recorded using Cu Kxcex1 radiation, is shown in FIG. 1. It was recorded using a computer-controlled Siemens D 500 powder diffractometer.
The dye of the formula I is known, its preparation is described, for example, in Japanese Patent Specification 74/37,931. It involves diazotizing 2-cyano-4,6-dinitroaniline and coupling the resulting diazonium salt onto 3-[bis(2-methoxyethyl)amino]acetanilide. This results in the formation of a dye in a slightly crystalline, unstable modification (xe2x80x9cxcex11 modificationxe2x80x9d) whose X-ray diffraction pattern (Cu Kxcex1 radiation) is shown in FIG. 2.
An alternative synthesis is described in EP-A-545,161. It involves preparing the dye of the formula I by the so-called cyano exchange method by reacting, for example, 2-acetylamino-4-[bis(2-methoxyethyl)amino]-2xe2x80x2-bromo-4xe2x80x2,6xe2x80x2-dinitroazobenzene with cyanide. In this reaction, it is obtained in a crystal modification (xe2x80x9cxcex12 modificationxe2x80x9d) whose X-ray diffraction pattern (Cu Kxcex1 radiation) is reproduced in FIG. 3 and which is characterized by lines at the following diffraction angles 2xcex8
high-intensity lines: 6.1, 18.0, 22.0, 24.3, 25.6, 26.3,
medium-intensity lines: 9.9, 12.5, 13.2, 16.
This crystal modification is also unstable.
However, powder and liquid preparations prepared from these unstable crystal modifications of the dye exhibit substantial technical defects, in particular during handling, but also already during preparation and also during dyeing of textile polyester materials. Technical defects during handling are observed, for example, when these preparations are redispersed, i.e., when they are incorporated in dyeing liquors and printing pastes. However, problems arise especially when these preparations are used in redispersed form in modern dyehouses.
The dye can be used without problems if it is present in the xcex2 modification according to the invention. This xcex2 modification allows higher space-time yields to be achieved when preparing powder preparations and dyeings are produced on piece goods and wound packages which are speckle-free and free of dye deposits, that is, are homogeneous.
The xcex2 modification according to the invention can be obtained by heating the a modifications in aqueous phase to temperatures of 70 to 150xc2x0 C., preferably 80 to 130xc2x0 C. This heating usually takes place in aqueous suspension, advantageously with stirring. If the temperatures to be applied are above the boiling point of the aqueous phase, the conversion into the xcex2 modification is carried out in sealed vessels, for example autoclaves. Heating is carried out for a time sufficient for converting the a modification completely into the xcex2 modification.